Increase In COP Research Articles

Energy and exergy analysis of a two-stage cascade vapor compression refrigeration system with modified system configuration

This study proposes a modification to the two-stage cascade vapor compression refrigeration system by adding internal heat exchangers that function as subcoolers and desuperheater. The influence of each internal heat exchanger proposed on the exergy destruction rate, exergy efficiency, compressor power consumption, ECOP, and COP of the system was investigated. Additionally, various refrigerant combinations were considered as working fluids to evaluate which combination is the most suitable for the proposed system. Mathematical models based on the principles of thermodynamics were established in Engineering Equation Solver (EES), a software used for energy and exergy analysis. The results reveal that the addition of specific internal heat exchangers causes either an increase or decrease in overall system performance, depending on the type of refrigerant combination used. Consequently, there exists an optimal system configuration for each refrigerant combination. Compared with the conventional two-stage cascade refrigeration system, the optimal system configurations in the present study exhibited higher overall system performance. A maximum increase in exergy efficiency, ECOP, and COP of 7.31 %, 9.8 %, and 7.3 %, respectively, can be observed with the refrigerant combination R450A/R404A. Additionally, the results of the exergy analysis identify that the HTC compressor, condenser, LTC compressor, and cascade condenser are the primary contributors to the exergy destruction rate within the system.

Experimental results on a chiller using a CO2-DME mixture

CO2-DME (Carbon Dioxide-Dimethyl Ether) mixtures raised our interest in the last year as they promise to increase the COP with respect to pure CO2 while maintaining GWP = 1. The mixture can be non-flammable, as far as the percentage of DME is low. This study presents experimental findings obtained from a chiller system utilizing CO2-DME mixtures as the working fluid. The experimental investigation aims to analyse the performance and behaviour of the chiller under varying operating conditions. Key parameters, such as coefficient of performance, cooling capacity, and thermodynamic behaviour are examined to assess the suitability and efficacy of the CO2-DME mixture in chiller applications. The experimental results shed light on the thermodynamic characteristics and operational considerations of the chiller system, providing valuable insights into the potential of CO2-DME mixtures for environmentally friendly, safe and energy-efficient cooling technologies. The experimental findings indicate a maximum increase in COP of 15 % for a mixture comprising 10 % DME, with a condensation temperature of 35 °C. Furthermore, the cooling capacity is reduced only by 7 % in the worst case.

A visual defrosting control method for air source heat pump system based on machine vision

Frosting on the coil of the air source heat pump (ASHP) significantly affects its performance. An accurate defrosting control method is essential for the ASHP unit. Here, a novel visual defrosting control method based on machine vision is proposed. The frosting degree of the unit is accurately obtained by frosting image analysis with digital image processing to achieve on-demand defrosting. Fractal dimension, frosting degree coefficient, and single-circuit frosting evenness value are introduced to evaluate the frosting degree of coil images. The image acquisition point is determined with confusion matrix. Based on this, the logical framework of the proposed method is constructed, in which the defrosting starting and termination conditions are set as the moment when the frosting degree coefficient is equal to 0.9 and 0, respectively. A multi-variable model of fractal dimension is fitted, which improves the generalization of the proposed method. Finally, comparative experiments are conducted between the proposed method and the traditional temperature-time (T-T) method in typical cases. The results indicate that the COP and total heating capacity increase by 11.8 %, 10.6 % and 7.4 %, 6.6 %, respectively, and the defrosting frequency is reduced by 75 %, which demonstrates dominant advantages of the proposed method in accuracy, energy-saving, and indoor thermal comfort management.

Performance simulation of metal hydride based flat coupled beds with differential sizes for cooling/heating

Hydrogen being the most abundant element in the universe has great potential towards decarbonisation at the point of usage. Metal hydride-based heat pumps/ refrigerators are getting increasing acceptance due to its environment friendliness and safety. These systems operate over a wide range of temperatures and pressures. The thermal performance of these systems depends mainly on the reaction kinetics of the alloy pairs. Difference in the reaction rates of the alloy pairs, causes a certain amount of hydride to go unutilized during cycling. While a variety of reactor designs and heat transfer enhancements are suggested, optimal thermal design of coupled beds with minimum weight and cycle time remains a major challenge. In the present study, numerical simulation of differently sized coupled beds based on the alloy pair of LaNi4.7Al0.3 - La0.8Ce0.2Ni5 is carried out using COMSOL Multiphysics® commercial code. The effect of mass ratio on the hydrogen pressure and hydride concentration in the coupled bed is highly significant. It is found that 12 % increase in COP (0.28) is obtained for a cycle time of 260 s and mass ratio of 1.5. This corresponds to minimum alloy weight ensuring effective transfer of hydrogen across HT-LT reactors.

Case study of variable speed photovoltaic direct-driven ice-storage air conditioning system in Wuhu

In order to meet the contradiction between the growing demand for refrigeration and energy scarcity, this paper proposes a novel photovoltaic ice storage air conditioning system. The system is directly driven by a photovoltaic array. The performance of the system was studied through experiments, and a mathematical model was constructed for comparative verification. The experimental results show that the daytime cooling COP of the system is 1.18, the photovoltaic conversion efficiency is 6.29 %, and the nighttime cooling time is 9 h, which basically meets the cooling needs. Through simulation, the performance of the system was optimized, and the results showed that an increase in the initial water volume and temperature of the ice storage tank would increase the cooling COP; the spacing between evaporator pipes increased from 45 mm to 60 mm, resulting in a 7.11% increase in COP; the optimal spacing between evaporator fins is 9 mm, which improves performance by 9.11 %.

Embracing the Evaporative System in Air Conditioning Technology for Efficient Cooling Solutions

The use of an air conditioning system requires large amounts of electrical energy to carry out repeated vapor compression cycles. The use of an evaporative cooling system in this research is by spraying condensate water on the condenser, which is one solution to absorb condenser heat. Another thing that can be done is to reset the fan speed on the condenser to cool it. Resetting the fan speed on the condenser can also help improve AC performance and reduce electrical energy use. The test was carried out by modifying the 1 Pk R-410a AC split condenser by installing 6 nozzles, 2 rows of 3 columns and a DC pump to spray water on the condenser. The independent variables of this research are spraying position and fan speed. The result obtained from the research is an increase in COP by 35% and a reduction in electrical power usage by 15% by using additional water spray with a nozzle behind the condenser both when blowing the full blower and when the blower blowing speed is reduced by 75%. The use of evaporative systems in air conditioning technology is a promising solution to achieve sustainable and efficient cooling solutions.

Theoretical investigation of a novel distributed compression cycle for CO2 trans-critical system

Efficient cooling of the trans-critical CO2 vapor compression cycle to improve system performance is a crucial research topic. This paper proposes an innovative approach called distribution compression (DC) to achieve the same effect as subcooling in a trans-critical CO2 thermodynamic cycle. In the DC system, the trans-critical CO2 at the gas cooler outlet is no longer further cooled but subjected to a secondary pressure boost and releases heat in ordinary heat sink conditions as in the gas cooler. Thermodynamics calculated the change in performance of the DC system under different working conditions with different secondary boost ratios. The optimal secondary boost ratio related to evaporation and gas cooler outlet temperatures under optimal discharge pressure based on the baseline system was obtained. The results show that compared with the baseline system, the DC system can effectively improve the system performance, and the maximum refrigeration COP increase is between 8.2 % ∼ 10.76 %, and the maximum heating COP increase is between 5.37 % ∼ 6.34 %. The cooling or heating capacity increase can reach up to about 26 %. The ideal secondary boost ratio in a DC system is not highly demanding, and the increased input power for the second input power for the secondary boost will not exceed 20 % relative to the baseline system. Comparing the COP of the DC system over current systems that only use a single subcooling technology still shows advantages. The DC system provides a new idea to improve the performance of the trans-critical CO2 vapor compression cycle.

Simulation and performance research of absorption heat pump unit based on R134a-DMF working fluid pair

R134a-DMF absorption heat pump unit is an energy-saving heat pump unit that can utilize renewable energy, and has great potential in the refrigeration and heating fields of urban and rural areas. The purpose of this article is to conduct in-depth research on the dynamic characteristics of absorption heat pump units based on R134a-DMF, a new working fluid pair. A mathematical model of the thermophysical properties of the R134a-DMF working fluid pair and the mathematical models of various components of the heat pump unit are constructed. This paper constructs a simulation program, and uses the Control variates to study the change trend of the Coefficient of performance of R134a-DMF absorption heat pump unit affected by the generator outlet concentrated solution temperature, condenser air volume and temperature rise, and chilled water outlet temperature. The results indicate that the established mathematical model for thermophysical properties and the unit model are both accurate models, which can provide guidance for the actual operation and optimization of R134a-DMF absorption heat pump units. Through simulation, it can be concluded that for the three combined forms of R134a DMF (3:2), R134a DMF (1:1), and R134a DMF (2:3), the average increase in COP and refrigeration capacity is 0.85 %, 0.39 %, and 0.42 % for each 1 °C increase in the outlet concentrated solution temperature of the generator, respectively, and the growth rate is relatively slow. The larger the proportion of refrigerant in the binary solution, the greater the COP of the unit under the same operating conditions.

Performance enhancement and environmental analysis of vapor compression refrigeration system with dedicated mechanical subcooling

The primary focus of this study is on the energy, exergy, and environmental (3E) analysis of a dedicated mechanical subcooled vapor compression refrigeration (DMS-VCR) system for applications involving commercially available water chillers that employ R134a (in both subcooler and main cycle). For a cooling capacity of 100-kW water chillers, the mathematical model of the DMS-VCR system is built to determine the performance parameter of the system. The DMS-VCR system reduces electricity usage by 15.52% and increase in COP by 9.5%, which results in a significant reduction in CO2 emissions of about 15.55%. When compared to equivalent vapor compression refrigeration system (VCRS), the system’s exergetic efficiency is also increased by 8%. Since the computer simulation results will undoubtedly give design engineers a better option, the subcooling and superheating of the vapor compression refrigeration system become alluring in this study. Consequently, the DMS-VCR system performs better as per the combined 3E study.

A proposal for a non-flammable, fluorine-free, CO2-based mixture as a low TEWI refrigerant

A thermodynamic analysis of a simple inverse cycle is proposed as a means to evaluate mixtures of CO2 and four organic substances (propane, iso-butane, dimethyl-ether and propylene) as working fluids. The cycle features an internal heat exchanger and is designed to cool down a finite heat capacity flow on the cold side. The analysis covers trans-critical as well as sub-critical cycles. For each organic substance, the composition is optimized. Flammability is accounted for. Dimethyl-ether (DME) turns out to be the best option, as a relatively low mass fraction of this compound is sufficient for reaching the thermodynamic optimum. In this way, a simple, sub-critical refrigeration system may be obtained featuring a non-flammable working fluid and a significant COP increase with respect to pure CO2.

High temperature transcritical CO2 heat pump with optimized tube-in-tube heat exchanger

CO2 heat pump operating on tanscritical cycle is widely adopted to provide hot water whose temperature is as high as about 90 °C. Recently, it is recognized that high temperature heat pump has promising application in various industries such as papermaking, chemical, automotive, and metallurgical. Then, in this paper a model of a tube-in-tube heat exchanger was established, and its parameters including tube length, fin height, fin number, fin thickness and helix angle are optimized through simulation to enhance its heat transfer performance, Accordingly, a transcritical CO2 heat pump system which can provide about 100 °C pressurized hot water is proposed and investigated. Subsequently, an experimental setup is constructed and operated under ambient temperatures, inlet and outlet temperatures of cooling water to demonstrate the feasibility of obtaining hot water at 100 °C. When the ambient temperature and inlet water temperatures are 40 °C and 9 °C respectively, the highest COP of 3.64 is obtained in experiments and the corresponding simulated value is 3.87. When the ambient temperature is 40 °C, inlet and outlet water temperatures are 9 °C and 85 °C, respectively, the study recorded the highest COP of 4.47. It is worth noting that both the heating capacity and COP increase with rising ambient temperature.

Experimental and theoretical investigation on the energetic and economic performance of low-temperature cascade air-source heat pump considering the effects of cascade heat exchanger

Air-source heat pump (ASHP) is wide used for its carbon neutrality. In cold climates, to ensure good efficiency and stability, cascade ASHP is a promising technology for high-temperature water application, such as space heating boilers retrofit. Cascade heat exchanger is a key component in cascade ASHP. However, there are few experimental studies on the effects of its area on system energetic, economic, and environmental performance under low ambient temperature and high water-supply temperature, which is essential for system improvement and application. Focusing on this research gap, this paper carries out experiment of cascade ASHP system performance and conducts analysis on system suitability based on recurrent Neural Network. To fairly compare system performance in different cold climate cities with different cascade heat exchanger areas, conversion factor γ is proposed and used in the analysis. The results show total energy consumption of compressors decreases the most in New York (21498 MJ) and least in Shenyang (9414 MJ) as cascade heat exchanger areas increases from 0.6 to 1.6 m2. Similarly, the system in New York shows the most significant reductions in carbon emissions (2701 kg/year) and total annual cost (225$), while that in Shenyang shows the smallest drop of 1277 kg/year and 118$, indicating it’s more meaningful to increase cascade heat exchanger area in a city with longer heating period. The results also show the COP varies significantly across cities, strongly correlated with average ambient temperature. The increase in COP slows down when cascade heat exchanger area reaches 1.6 m2, indicating even larger area is not recommended. These findings provide a foundation for improving and applying cascade ASHP in cold climates.

Examining the efficacy of cooling pad technology to address increasing building cooling demand in Latvia

Over the past decades there has been a strong evidence of a temperature rise across the world that has led to a growing concern of more extreme weather patterns and regular seasonal heat waves globally. As such, building occupants are at a continuously growing risk to overheating exposure inside the premises throughout the warm season of the year. This study investigates the utilization of cooling pad technology as a potential solution to enhance cooling efficiency. Compared to traditional cooling methods, the implementation of cooling pads leads to significant reductions in temperature and enhanced humidity control, while consuming relatively lower amounts of energy. The study contains a comprehensive analysis of the climatic conditions in Latvia, focusing on temperature and humidity variations throughout the year over the last decade in three cities–Riga, Daugavpils and Liepaja, that extensively represent the scope of climatic variations across Latvia, featuring coastal and continental climate patterns. This study aims to evaluate the effectiveness and suitability of cooling pad technology in Nordic climate, focusing on three Latvian cities. The novelty of the study lies in its analysis of cooling pad technology’s effectiveness in Nordic climatic conditions in addressing the increasing cooling demand. The paper examines the fundamental principles behind cooling pad technology, its impact on chiller performance, and its ability to optimize the cooling process. The utilization of cooling pad technology as an effective means to enhance cooling efficiency across the building stock to improve occupant comfort level and IEQ is highlighted. The results demonstrate 5.47% COP increase during average summer temperature conditions, and 17.78% COP increase in peak summer temperature conditions after implementation of cooling pads. This study contributes to the existing knowledge on cooling technologies, offering practical recommendations for the implementation of cooling pad systems use in Latvia and across the wider Nordic region, which is experiencing the gradual rise in summer temperature and humidity level.

Performance Analysis of Using Hydrocarbon Mixed Refrigerant R32-R290 as an Alternative to R410A in Reducing the GWP Value of Household Split Air Conditioners

An experiment conducted on the mixed refrigerant R32-R290, which is used in household air conditioners, is aimed at reducing the global warming potential and improving the environmental CO2 quality. This mixed refrigerant has a lower GWP and density than the refrigerant R410A. This research was conducted to determine the effect of the increased mass charge of R290 as a mixed refrigerant on the performance of split air conditioner. Three percentage compositions of 20%/80%, 25%/75%, and 30%/70% were applied as a change to R410A in household air conditioners. For the testing equipment condition, the air inlet evaporator and condenser temperatures were 17°C (300 K) and 35°C (308 K), respectively. In the first performance test, refrigerant mass charge of 100% (340 g) was employed. In the second test, mass charge of 40% (136 g), 50% (170 g), and 60% (204 g) were utilized for mixed refrigerant R32-R290. Compared to the performance of R410A, the experimental results for the mixed refrigerant indicated a 0.5% increase in the air outlet evaporator temperature, a 1% decrease in the air condenser outlet, a 29.7% increase in the average COP, a 12.9% decrease in the electrical power consumption from 545.12 W to 474.7 W, a 9.1% increase in the average exergy destruction average from 401.5 W to 441.6 W, and a 19.7% increase in the average exergy efficiency from 24.2 W to 30.1 W.

Energetic, exergetic, environmental and economic (4E) analysis of a direct-expansion solar-assisted heat pump with low GWP refrigerant

The main objective of this study was to develop, validate and apply a mathematical model for analysing the performance of a Direct-Expansion Solar-Assisted Heat Pump (DX-SAHP) based on the energetic, exergetic, environmental, and economic (4E) analysis. The DX-SAHP produces domestic hot water using the low-global-warming-potential (GWP) refrigerant R290 which is the best candidate for energy, environmental, and logistical performance among six previously evaluated candidates (R152a, R744, R1234yf, R1234ze(E), R170, R290). Environmental and economic analysis are founded, respectively, on Total Equivalent Warming Impact (TEWI) and payback. The proposed model was validated using an experimental test bench developed for this study. Using the validated model, simulations were conducted to investigate the influence of geometric parameters (evaporator area and condenser length) on the system's performance. The results show that increasing the evaporator area yields the greatest increase in COP (117 and 80.4%) and the greatest decrease in TEWI (53.8 and 44.5%) for final water temperatures of 45 and 65°C, respectively. However, this also leads to a drop in collector efficiency (58.7 and 60.8%) but results in a shorter payback time (6.06 and 3.87 years) for final water temperatures of 45 and 65°C, respectively. The greatest increase in exergy efficiency (11.5 and 14.1%) occurs with an increase in condenser length for final water temperatures of 45 and 65°C, respectively.

Analysis of thermophysical properties and performance of nanorefrigerants and nanolubricant-refrigerant mixtures in refrigeration systems

Nanorefrigerants and nanolubricants have improved refrigeration system productivity. This research theoretically investigates Multi-Walled Carbon nanotubes (MWCNTs) and Copper Oxide (CuO) nanoparticles at volume fractions of 0.5, 1, and 2% in R152a and R134a refrigerants with Polyester (POE) lubricant. Thermophysical characteristics and COP will be examined. As volume concentration increases, thermal conductivity, density, and viscosity enhance, while specific heat capacity diminishes. However, after the nanoparticle volume concentration surpasses its optimal value, the specific heat capacity declines, potentially resulting in a reduction in cooling capacity. This inverse link suggests that in order to perform at one's best, a balance must be achieved. Despite this, nanorefrigerants and nanolubricant-refrigerants have improved COPs. Nanoparticle thermal conductivity is the main reason. R152a-based nanolubricant-refrigerants have greater COP values than R134a-based ones. R152a-MWCNTs-based nanorefrigerant had a maximum COP increase of 27.63% over R152a.In conclusion, nanorefrigerants made of R152a and Polyester (POE) lubricant are a good refrigeration option. Due to their high performance and environmental friendliness, these materials improve efficiency and energy consumption more than R134a.

Thermodynamic investigation of a direct-expansion solar assisted heat pump with evacuated tube collector-evaporator

Direct-expansion solar-assisted heat pump (DX-SAHP) system has proven to be an effective energy-saving application. In view of the low output temperature and large radiant heat loss in the conventional flat plate type DX-SAHP, this paper proposed an evacuated tube type DX-SAHP system to realize performance enhancement. Then, an experimental prototype of the proposed system was originally designed and constructed in laboratory. Moreover, the thermodynamic performance of the system was thoroughly investigated and analyzed under different operating conditions. The results indicate that the thermodynamic performance of the proposed system is sensitive to the solar radiation intensity and collector area. About every 100 W m−2 increase of solar radiation intensity, the COP increases nearly 1.70% and the heat collection efficiency decreases nearly 8.44%. Increasing collector area (from 1.5 m2 to 3 m2) results in 8.81% increase of COP and 22.47% reduction of the system exergy efficiency. A higher circulating water temperature can effectively improve its exergy performance and higher ambient temperature can enhance its energetic performance. Additionally, the optimization of the evacuated tube collector-evaporator should be preferential to improve the system due to large exergy destruction and low exergy efficiency.

Critical assessment of R410A alternatives for mini-split air conditioners in the Egyptian market

In this study, the potential implementation of three different low-GWP refrigerants (R32, R452B, and R454B) as replacements for R410A was investigated. The study was performed using a simulation tool developed by the authors called RACHP-Lab, which is a vapor compression system simulation tool developed based on physics-based simulation for typical mini-split air conditioners. The simulation study was carried out and validated using experimental performance data of 10 different air conditioning units available in the Egyptian market. The units included fixed-speed or variable-speed compressors and operated in cooling or heating modes. Drop-in replacement with the new refrigerants was carried out. For R32, the capacity increased between 4.9% and 13% for cooling cases, and 6.3% and 12.4% for heating cases. However, COP did not improve in all cases. For R452B and R454B with direct replacement, the capacity nearly remained the same, with an increase of COP between 1.6% and 8.0%. Soft optimization was also conducted on cooling cases where compressor suction superheat, condenser subcooling, and compressor volumetric speed were optimized to maximize COP while maintaining the original capacity of R410A. R32 showed an improvement of COP over R410A between 4.6% and 15.5%, while for R452B and R454B between 2.2% and 13.2%.

Role of Non-Adiabatic Capillary Tube in Water Cooler Performance

In this paper, a numerical model of a capillary tube is developed. The considered expansion device is placed against the suction line at the inlet of the compressor. Wrapping the capillary tube around the suction line allows heat to be recovered by superheating the refrigerant leaving the evaporator. This increases the degree to which the fluid is superheated, preventing liquid droplets from entering the compressor and causing damage. The open-source software PYTHON is used for modelling the non adiabatic capillary tube, and the results are validated by comparing them with experimental tests. This study demonstrates that an accurate contact of the capillary tube with the suction line affects the superheating of the compressor inlet fluid by increasing its temperature by up to 5 degrees and produces an increase in COP of 3–4%. On the other hand, the length of the capillary tube affects the flow rate of the refrigerant circulating in the cycle; in particular, it is noted that a 300% increase in the capillary tube length leads to a decrease in the refrigerant flow rate of up to 50–60%.

EFFECTS OF VAPORIZATION TEMPERATURE AND SUB COOLING VARIATION ON THE PERFORMANCE OF A VAPOUR COMPRESSION REFRIGERATION CYCLE WORKING WITH R134A, MET ON NEWER SHIPS

R134a is a refrigerant met in several marine refrigeration applications, such as fishing vessels, passenger and cargo ships. In 2014, 26% of the international commercial fleet was using R134a. Although R134a shows a null Ozone Depletion Potential, it has a quite high Global Warming Potential (1300). R134a is a greenhouse gas and, even if it is present on newer ships, the future will be marked by its replacement with substitutes having low GWP. Still, because its GWP is less than 2500, R 134a will continue to be used. Due to the fact that vapour compression refrigeration systems are dominant on board the ships and knowing that these technologies are high energy consumers, analysing their performance in the contemporary energetic context, is imperious required. This paper presents a theoretical analysis of a single stage vapour compression cycle, working with R134a, based on the laws of thermodynamics. The analysis will reveal the influence of the evaporator temperature on the Coefficient of performance and on exergy efficiency, and also the influence of sub cooling on these two efficiency terms, on the refrigerant mass flow rate and compression rate. It was considered a variation of the evaporator temperature in the range (-40÷ -10)oC and of the sub cooling in the range (0÷10)oC. The increase of the evaporator temperature will contribute to a COP increment (50%) and an exergy efficiency decrease (34%). The sub cooling will lead to both COP and exergy efficiency increase (11%). Higher sub cooling degree will provide an increment in the refrigerant mass flow (18%) and a decrease of the compression rate (76%) meaning lower work consumption at the compressor.